The skin is an organ from to the tegumentary system (along with hair and hairs, nails, sweat and sabeceous glands) and the main functions of which are to protect the subjacent tissues, regulate the somatic temperature, maintain water homeostasis, nutrient reserve and also contain sensitive nerve terminations.
This organ has three layers or strata: epidermis (most superficial), dermis (intermediary) and subcutaneous hypodermis (technically external to the skin, but functionally related). There are also various organs attached, such as hair follicles, sweat and sebaceous glands.
The epidermis is a stratified epithelium, that is, a tissue formed by juxtaposed cells, which is separated from the dermis by a basal membrane. It comprises five layers or strata: germinative, spinous, granular, lucid and corneum. The purpose of its constitution and structure is to keep the balance of the surface of the body, preventing physical damage (e.g. rigors of temperature, humidity and drying, traumas, ultraviolet radiation), chemicals (e.g. alkalis, acid soaps) and biological (e.g. microorganisms, such as virus, fungi and bacteria).
Most of the cells of the epidermis are comprised of keratinocytes. In constant renewal, keratinocytes become corneocytes. In this natural process, each keratinocyte which is produced in the basal layer migrates to the surface, also producing on this trajectory lipids and proteins, of which the most important is keratin. As soon as it becomes full of keratin, the keratinocyte loses its nucleus, also changing its physical form (from cubic to hexagonal and flattened), becoming a corneocyte, which becomes evermore peripherical, and ending up flaking and falling off. The corneocytes constitute the outermost layer of the epidermis, called corneum layer or stratum corneum.
Over recent decades, many studies have been carried out with the objective of elucidating the structure, workings and role of the stratum corneum.
The ‘brick and mortar’ structural model proposes that the stratum corneum is a structure in which the cells of the corneocytes are arranged in successive layers, and the link between them is realized by the intercellular ‘mortar’, which comprises a lipidic matrix organized in bilayers. It may include ceramides, cholesterol and certain fatty acids which have relatively large hydrocarbon chains and unsaturations.
The lipids of the stratum corneum are contained in cytoplasmic organelles, called lamellar bodies, which can be found in the upper layer of the spinous stratum. The lamellar bodies are formed in the endoplasmic reticulum and, by way of exocytosis; its content is expelled in the intercellular space, in the transition from granular stratum to stratum corneum.
The corneocytes jointly with the lipids produced in the epidermis and sebaceous glands (epidermic and sebaceous, respectively), in addition to the water with mineral salts originating from sudoresis (electrolytes), form the cutaneous barrier, fundamental to the integrity of the protective function of the skin, as it keeps the pH ideal, controls the diffusion of molecules or gases such as oxygen and carbon dioxide and homeostasis of the water levels.
The intercellular lamellar lipids are organized in three phases: crystalline, hexagonal and orthorhombic. The predominant phase is the orthorhombic. This organization provides an effective barrier for the passage of water and any disturbance in this organization results in undesirable drying conditions (dry or unhydrated skin).
Hydrated skin, in turn, is highly desirable because it is perceived as soft-to-the-touch skin, having malleability, without opacity or flaking.
It is recognized that not all emollients are the same in terms of their capacities to humidify the human stratum corneum (hydrate the skin). Skin humidifiers can be classified as moisturizers or occlusion agents, according to its action mechanism.
Moisturizers are small, hygroscopic molecules that penetrate into the stratum corneum and subsequently act as moisturizers. In order that moisturizers may act by this mechanism, they must be not only hygroscopic, but must penetrate the skin. Glycerin is the most well-known moisturizing ingredient. It is extremely hygroscopic and attracts water molecules. Besides this one, glycols and other polyhydroxy molecules, such as propylene glycol, butylene glycol, glucose, sucrose and sorbitol also act by this mechanism.
The second group, occlusion agents, generally involves lipophilic materials which contain long, unbranched alkyl chains, without double bonds. It is an essential requirement that these agents be substantive with the skin and capable of aligning their hydrocarbon tails, to create an occlusive layer on the surface of the skin. This layer prevents water evaporation from the skin and, therefore, the water content of the stratum corneum is increased. The moisturizers that act by this mechanism are generally hydrocarbons, such as mineral oil and petrolatum.
Besides using ingredients having a moisturizing nature, certain complex cosmetic compositions have been developed with the purpose of providing the perception of hydrated skin. Many of them use specific active principles, sometimes of a high commercial value, as well as formulation strategies that require the use of a large number of raw materials and strict quality control.
In terms of structuring, the most common formulations are of the oil-in-water type, for example, such as described in patent documents WO05108383 and U.S. Pat. No. 5,925,364. There are also formulations of the oil-in-glycerin type, for example, such as those described in patent documents WO0056346, U.S. Pat. No. 6,342,238, WO06119042 and WO0224152. The latter, designed for the transport or permeation of active compounds by way of layers of the skin or merely its protection against degradation, does not remedy the drawbacks of skin drying either immediately and/or in the long run.
As well recognized in the state of the art, cosmetic compositions are generally prepared with the use of anionic, cationic, amphoteric and/or non-ionic surfactants. These compounds are described, for example, in the work entitled Sufactants, by Martin Rieger, published by Society of Cosmetic Chemists in 1997 (incorporated as a reference herein).
Esters derived from carbohydrate constitute a subclass of non-ionic surfactants. These surfactants do not have a load at the pH levels normally used in the cosmetics sector. Owing to their characteristics, they are used in isolation or in combination, in different formulation strategies, for example, as described in documents WO05108383, U.S. Pat. No. 5,925,364, JP10231229, US 20070286835 and WO0929046. In general, the technique teaches their use in low concentrations, between 0.2-2.5% in relation to the total weight of the composition.
Further, in relation to degradation, it is important to consider that most compositions of the oil-in-glycerin type, containing non-ionic surfactants or not, use ingredients that present unsaturations, that is they are more susceptible to oxidation and, therefore, less stable, for example, vegetable oils, ceramides, cholesterol, etc. In general, these compositions seek identity with the composition of the skin which, for example, contains ceramides as one of its main components.
Wiechers et al. (Skin Moisturization, second edition, Informa healthcare, USA, 2009), introduced a third action mechanism associated to skin moisturization, that of breakdown of the lipidic barrier.
The experiments revealed that different distributions of water in the stratum corneum, as a consequence of applying two emollients, could have been caused by different interactions with the organization of the skin lipids.
Applying moisturizers to the skin not only attracts and maintains water, but interferes in the configuration of the lipidic phases.
It was concluded that the dysfunction in the workings of the barrier of the stratum corneum is at the heart of many problems associated to skin with altered balance. Therefore, normalizing the structure of the lamellar lipids would therefore be a strategy to be pursued.
Thus, the correct water gradient can only be maintained in the long run if the skin barrier is completely functional. Therefore, if the lipidic phase is stabilized in orthorhombic, then the skin barrier is appropriately restored and all the symptoms of skin with altered balance will be reversed, such that the vicious cycle of imbalance can be broken. Although molecules such as glycerin and mineral oil may temporarily solve the drawbacks of dry skin, they knowingly do not act on the causal problem, such as modulating the behavior of the lipidic phase, restoring its orthorhombic organization.
Even though this new mechanism has been established, there are no products efficient at stabilizing the lipidic phase, restoring the orthorhombic structuring thereto.
Stabilizing the lipidic phase in orthorhombic leads to an increase in water retention of the stratum corneum by natural means; that is the skin barrier will be responsible for the necessary moisturizing response, by means of efficient control of the water flow. This leads to a more efficient and long-lasting response to the problem of dry skin, as well as sensorially more pleasurable than that which the use of traditional moisturizing agents may offer only as a temporary means, due to its adverse effects in disorganizing the lipidic layer.
So the need remains for cosmetic compositions that act directly in reorganizing the lipidic layer in orthorhombic and, consequently, providing a prolonged effect, also avoiding the use of ingredients susceptible to oxidation and the direct action in deeper layers of the skin (permeation).